The present invention relates to a battery pack comprising, e.g., a lithium-ion secondary battery.
Currently, in many electric devices operated using a battery, a rechargeable secondary battery is applied, and is discharged and charged repeatedly. In a battery pack used in these devices, as charging and discharging operations are repeated, the secondary battery deteriorates, and even the battery in a full charge state cannot achieve a discharge capacity, or service capacity, of a new battery.
FIG. 1 shows the results of a cycle test in which a cycle of such charging and discharging operations is repeated that a lithium-ion secondary battery is charged at 4.1 V or 4.2 V, and allowed to stand in this high voltage state for a predetermined period of time, and then discharged until the voltage of the battery becomes 3.0 V. As can be seen from FIG. 1, in the lithium-ion secondary battery allowed to stand at 4.1 V, a remarkable reduction of the discharge capacity is not found even after the 300th cycle, whereas, in the lithium-ion secondary battery allowed to stand at 4.2 V, the discharge capacity begins to be reduced after the 200th cycle, and the discharge capacity is rapidly reduced after the 250th cycle. In this way, in the battery pack using, particularly a lithium-ion secondary battery as a secondary battery, there are some cases where the battery is likely to rapidly deteriorate when the secondary battery is allowed to stand for a long time in a state such that the battery voltage remains high.
Charging of a lithium-ion secondary battery is generally conducted by a method using constant current charging and constant voltage charging in combination. This method is employed to avoid a danger that the lithium-ion secondary battery in the full charge state continues to increase in battery voltage during the charging and the battery is consequently in an overcharge state and suffers temperature elevation or ignition. The constant current and constant voltage charging is performed as follows. At a battery voltage in a range of a predetermined voltage (e.g., Vb=4.1 V) or lower, constant current charging is conducted at a predetermined current (e.g., Ib=500 mA). At a battery voltage of larger than 4.1 V, a power source unit is constant voltage control-operated, so that the charging current Ib is gradually reduced. Then, the battery voltage Vb is increased to a predetermined output voltage (e.g., Vo=4.2 V) of the charge power source unit, thus completing the charging.
A method for detecting a state-of-charge of the secondary battery is described. As a method for detecting a state-of-charge, a method of a current detecting system and a method of a ΔV detecting system are known. The method of a current detecting system is a method such that, utilizing the reduction of the charging current Ib during the constant voltage control at the end of the charging, the charging current Ib is converted to a voltage Ex by means of a resistance and the voltage Ex is compared to a detecting voltage Ei to detect a state-of-charge. When the voltage Ex is equal to the detecting voltage Ei, the battery is judged to be in the full charge state. The resistance has a loss caused due to the charging current Ib, and therefore this method is effective for a device having a relatively small charging current Ib. The method of a ΔV detecting system is a method such that an output voltage Vo of a charge power source unit and a battery voltage Vb of the secondary battery are measured and, when a difference ΔV between them becomes a predetermined voltage (several mV), the battery is judged to be in the full charge state. The method of a ΔV detecting system detects a state-of-charge by measuring a voltage, and hence is effective for a device having a large charging current Ib.
When the full charge state is detected by the above detecting method, the charging is stopped. In the present specification, the term “full charge voltage” means a voltage at which a battery pack charged by a suitable charger is in the full charge state as mentioned above and the charging of the battery should be stopped.
A battery pack generally has a protection circuit including a charge/discharge control field effect transistor (FET) and an integrated circuit (IC) for monitoring the secondary battery and controlling the charge/discharge control FET. The protection circuit has an overcharge protection function.
The overcharge protection function of the protection circuit is now described. As mentioned above, safety of the lithium-ion secondary battery is secured by charging the lithium-ion secondary battery at a constant current and a constant voltage and at a charge control voltage equal to or lower than the rated voltage of the battery (e.g., 4.2 V). However, when a charger malfunctions or an unsuitable charger is used, there is a danger that the battery is overcharged. When the battery is overcharged and a battery voltage equal to or higher than the overcharge detection voltage (e.g., 4.325 V) is detected, the protection circuit turns off a charge control switching element, for example, a field effect transistor (FET), to cut off the charging current. This is the overcharge protection function. The overcharge detection voltage is a voltage at which the protection circuit cuts off the charging current.
Japanese Patent Application Publication (KOKAI) No. 2003-125540 describes that a battery pack in which protection function control means monitors a terminal voltage of a secondary battery, and, when it detects overcharge, it displays a warning to flow a self-consumed current, thereby lowering the voltage of the secondary battery to a safe voltage range as soon as possible.